This invention relates to apparatus for and a method of automatically biasing the superheat setting of a thermostatic expansion valve in a refrigeration system, an air conditioner, a heatpump, or other similar systems so as to maintain a low superheat setting for the thermostatic expansion valve and to maintain operation of the evaporator coil of the refrigeration system in a flooded or nearly flooded condition under a wide range of operating conditions, and yet so as prevent liquid refrigerant from entering the compressor.
Generally, a refrigeration system includes a compressor, a condenser coil, and an evaporator coil. Refrigerant vapor is compressed to a high pressure by the compressor and is conducted through the condenser coil where it is cooled to form a liquid under high pressure. This high pressure liquid is then adiabatically expanded through an expansion valve and is admitted into the evaporator coil where the refrigerant picks up heat from the surroundings of the evaporator coil. The heat absorbed by the evaporator coil transforms low pressure liquid in the evaporator coil into a vapor. The vapor is then conducted through the suction line of the refrigeration system and is returned to the inlet of the compressor.
Generally, it is not desireable that excessive liquid refrigerant be returned to the inlet of the compressor from the evaporator coil because this liquid refrigerant may dilute the lubricating oil in a typical hermetic compressor and thus cause damage to the compressor. Also, excessive quantities of liquid refrigerant in the compressor may damage certain of the compressor components, such as the compressor reed valves. On the other hand, in certain hermetic compressors, if not enough liquid refrigerant reaches the compressor, the windings for the motor for the compressor may not be sufficiently cooled thus also resulting in damage to the compressor.
The term "superheat" means raising the temperature of the refrigerant vapor above the temperature required to change the refrigerant from a liquid to a vapor at a specified pressure level. In many refrigeration systems, such as in domestic refrigerators, air conditioners and heatpumps, the evaporator coils are designed for complete evaporation of the refrigerant in the evaporator coil with only vapor being returned to the compressor via the suction line. This vapor is typically superheated in the last part of the coil so as to insure that the refrigerant flowing back to the compressor is always a vapor under all operating or load conditions. However, it is desireable that the majority of the length of the evaporator coil have liquid refrigerant therein (i.e., that it be wetted) because there is a much higher heat transfer coefficient between the coil and the liquid refrigerant than between the coil and the vapor refrigerant. Thus, a thermostatic expansion valve is typically used to regulate the flow of refrigerant through the evaporator coil so that under various operating conditions for the evaporator coil, the majority of the length of the coil will have liquid refrigerant therein, but yet the refrigerant flow is so limited as to insure that the evaporator coil does not become flooded and thus prevents the return liquid refrigerant (or excessive quantities thereof) to the suction side of the compressor. In other words, the thermostatic expansion valve automatically controls the flow of the refrigerant into the evaporator coil so as to maintain the superheat of the refrigerant leaving the evaporator coil at a predetermined level under various operating conditions.
In essence, a thermostatic expansion valve automatically maintains an ample supply of refrigerant to the evaporator coil without allowing liquid to pass into the suction line and the compressor. The operation of a thermostatic expansion valve typically depends on the superheated condition of the refrigerant leaving the evaporator coil. Generally, a short portion of the length of the evaporator coil is intended to have refrigerant vapor therein so that the temperature of the vapor exhausted from the evaporator is above the temperature corresponding to the evaporative pressure. This elevated refrigerant vapor temperature is referred to as the superheat and, for example, the flow of the refrigerant through the evaporator coil of a typical refrigeration system may be regulated by the thermostatic expansion valve to result in a superheat of about 5.degree.-10.degree. F.
In operation, with the thermostatic expansion valve set to a predetermined superheat operating setting, the thermostatic expansion valve will automatically adjust the quantity of refrigerant delivered to the evaporator coil so as to maintain a desired superheat condition of the refrigerant vapor exhausted from the evaporator coil. Upon the evaporator coil being exposed to a greater heat load, the heat absorbed by the refrigerant will increase thus increasing the superheat of the refrigerant exhausted from the evaporator coil. This in turn causes the temperature of the fluid in the thermostatic bulb controlling the thermostatic expansion valve to increase and to force the valve to a more open condition thereby allowing more refrigerant into the evaporator coil so as to reduce the superheat temperature. Conversely, if the load on the evaporator coil should be lowered, the superheat temperature of the vapor exhausted from the evaporator coil decreases and thus the temperature of the fluid in the thermostatic sensing bulb of the thermostatic expansion valve is lowered which in turn effects closing of the thermostatic expansion valve thereby to reduce the flow of refrigerant to the evaporating coil and also in turn increasing the temperature of the vapor exhausted from the evaporator coil. In all cases, however, the superheat level of the refrigerant exhausted from the evaporator coil is maintained near the preset superheat setting of the thermostatic expansion valve.
It is known that in a wetted evaporator coil, the temperature of the coil along its length decreases substantially linearly from the inlet to the outlet of the coil. However, at a point along the length of the coil where superheating of the refrigerant vapor begins, the temperature of the coil begins to rise rapidly, and this temperature rise continues to the coil outlet. This temperature inflection point is referred to as a notch. It will be appreciated that in a flooded coil (i.e., a coil which is wetted along its entire length), the temperature of the coil will uniformly decrease from its inlet to its outlet. However, in a starved coil in which the last portion of the coil is not wetted, the notch moves toward the inlet end of the coil and the superheated portion of the coil is not as efficient in removing heat from the surroundings as the wetted portion of the coil. Ideally, the superheat of the thermostatic expansion valve should be controlled so that the entire length of the coil is flooded or wetted under all operating conditions. However, control of the superheat and/or the amount of liquid refrigerant reaching the compressor of a heatpump or other refrigeration system is difficult to accurately control with known prior art thermostatic expansion valves.
Generally speaking, a heatpump is a refrigeration system having two refrigeration coils, one inside the building and one outside the building, either of which may be used as an evaporator coil or a condenser coil. In the cooling or air conditioning mode of the heatpump, the indoor coil is used as the evaporator and, while in the heating mode, the outdoor coil serves as the evaporator. A four-way selector valve is provided in the refrigerant lines between the coils so that the flow of refrigerant may be selectively directed to either of the coils for operation either as an evaporator or as a condenser coil. In the past, it has been difficult to select a fixed superheat setting for a conventional thermostatic expansion valve which provides satisfactory control of the superheat setting at all operating conditions and under both the heating and air conditioning mode. For example, such operating conditions as frost buildup on the coils, start up after a cold soak at low ambient temperatures, and defrost recovery have been difficult to accommodate with a fixed superheat setting in a thermostatic expansion valve. Also, system component failures (e.g., the failure of a evaporator or condenser fan motor) may impose even more severe demands on the thermostatic expansion valve to maintain an acceptable superheat setting so as to prevent damage to the compressor. With conventional thermostatic expansion valves, the superheat setting may only be changed by mechanically adjusting or varying the spring force exerted on the valve or by changing the fluid in the thermostatic sensing bulb so as to exert more or less pressure on the diaphragm at selected temperatures. However, it is not practical or desirable for the homeowner, for example, to make such adjustments to a heatpump or to an other refrigeration system in the home.
In an attempt to overcome the above-identified shortcomings of the prior art thermostatic expansion valves, electrically operable refrigerant expansion valves, such as shown in the co-assigned U.S. Pat. No. 3,967,781 to Kunz, have been employed in conjunction with a control system responsive to a parameter, such as the temperature of the lubricant in the sump of the compressor, or to ambient air temperature, so as to modulate the superheat setting of the electrically operable expansion valve. This control system is disclosed in the co-assigned U.S. Pat. No. 4,067,203 to Behr. The control system disclosed in the above-noted patent to Behr enabled the precise control of an electrically operable expansion valve so that the evaporator coil could be continuously operated in a flooded state (i.e., with liquid refrigerant in all parts of the coil) under varying load conditions, and yet prevented liquid refrigerant from entering the suction side of the compressor. It will be recognized that when the evaporator coil is maintained in its flooded or wetted state, optimum heat transfer (and thus optimum operating efficiency) for the refrigeration system results.
While the electrically modulated expansion valve, such as disclosed in Behr, U.S. Pat. No. 4,067,203, worked exceedingly well for its intended purpose, it did require the use of an electrically modulated expansion valve and the well-known thermostatic expansion valve could not be used. Thus, there has been a long standing need for a reliable and simple control system for altering the effective superheat setting of a conventional thermostatic expansion valve.